sysreset: syscon: Support value property

Per the DT binding, <mask> and <value> property can have either one
or both, and if <value> is missing, <mask> should be used, which is
what current U-Boot sysreset_syscon driver supports.

This adds support to the <value> property to the driver, and <mask>
semantics is updated to really be a mask to the value if both exist.

Signed-off-by: Bin Meng <bin.meng@windriver.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Pragnesh Patel <pragnesh.patel@sifive.com>